Early heart failure and hypoxia due to congenital heart disease frequently result in small infants with sparse body fat. Surgical correction is often followed by weight gain and increased adipose tissue. Recent human and animal research have shown that adipose tissue growth in early infancy is primarily due to a rapid increase in cell number. Thereafter, the rate at which cell number increases falls and tissue enlargement is produced mainly by increased cell size. In animals, nutritional deficiency during the early period of rapid cell division can lead to a permanent low number of fat cells irrespective of subsequent nutritional correction. Therefore, we postulate that underweight infants with heart failure and/or hypoxia will have subnormal numbers of fat cells unless surgically improved during early life. To test our hypothesis, total fat cell number, and cell size and function will be determined by methods employing small adipose tissue specimens obtained at cardiac catheterization and surgery. Patients will be studied serially over several years. The cardiac patients will be compared to patients without heart or metabolic disorders. The importance of hypoxia, heart failure, and surgery in respect to age will be evaluated.